This invention relates to a method for the preparation of commercial grade phosphoric acid having improved stability against sludge formation. More particularly, the present invention is directed to the stabilization of merchant grade phosphoric acid produced in accordance with the wet-process.
It is well known that ammonium phosphate fertilizer is mainly produced by the reaction of ammonia and phosphoric acid to produce ammonium phosphate. Ammonia for use in such reactions is readily available in comparatively pure form. However, phosphoric acid is available in several grades with varying degrees of purity. Because of the economics involved, wet-process grade phosphoric acid is ordinarily used in the production of fertilizer since wet-process acid is much less expensive that other grades.
Phosphoric acid is produced by the wet-process by acidulation of ground phosphate rock with sulphuric acid to produce a slurry composed of phosphoric acid, gypsum and impurities that are inherent in the phosphate rock. This slurry is filtered to eliminate a large part of the solid impurities leaving a filtrate of dilute phosphoric acid containing a small portion of solids as well as soluble impurities. The dilute phosphoric acid is then concentrated to commercial grade phosphoric acid usually a phosphoric acid containing a P.sub.2 O.sub.5 content of 54% by weight.
During the digestion step of the wet-process, that is, the acidulation of the phosphate rock with sulphuric acid, some of the impurities in the phosphate rock such as, for example, iron, aluminum, silicon, fluorine, sodium and potassium are put into solution. When the filtrate is concentrated to commercial grade phosphoric acid, however, additional solids are formed. Moreover, after extended periods of time the wet-process phosphoric acid has a tendency to undergo solid formation and precipitation and form sludge consisting mainly of iron and aluminum phosphates. A primary contributor to this sludge formation has been found to be the potassium impurity present. The potassium promotes the post-precipitation which characterizes wet-process phosphoric acid by combining either with the iron and aluminum impurities to precipitate them as the metal phosphates or with the fluorosilicates to precipitate them as potassium silica fluoride. These two types of compounds are the main constituents of the solid or sludge that forms.
Needless to say, this sludge formation which occurs with commercial grade wet-process phosphoric acid results in a loss of appreciable quantity of phosphoric acid which is recoverable only by involved and costly processing. In addition, the sludge formation presents other difficulties in that it tends to eventually settle to the bottom of the storage vessels and clog the pipe lines and other equipment such as that used in the manufacture of phosphatic fertilizers.
It is an object of the present invention, therefore, to improve the stability of wet-process phosphoric acid against sludge formation during storage or after prolonged periods of time.
Yet another object of the invention is to provide a wet-process phosphoric acid of improved stability that is characterized by a reduced tendency to precipitate solids and at the same time by a reduced P.sub.2 O.sub.5 content in the few solids precipitated.